Modifies generator and specs to look for openapi: 3.0.0 vs swagger 2.0

Renames rswag-api to rswag_api as that is preferred file naming convention in initializers per rubocop linting

rswag-api/lib/generators/rswag/api/install/USAGE

@@ -5,4 +5,4 @@ Example:
     rails generate rswag:api:install
 
     This will create:
-        config/initializers/rswag-api.rb
+        config/initializers/rswag_api.rb

rswag-api/lib/generators/rswag/api/install/install_generator.rb

@@ -7,7 +7,7 @@ module Rswag
       source_root File.expand_path('../templates', __FILE__)
 
       def add_initializer
-        template('rswag-api.rb', 'config/initializers/rswag-api.rb')
+        template('rswag_api.rb', 'config/initializers/rswag_api.rb')
       end
 
       def add_routes